The present invention relates to measuring the mechanical properties of moving sheet or web-like materials, and more particularly, relates to a system and method for detecting ultrasonic signals, in a non-contact manner, in moving sheet or web-like materials.
Ensuring quality control is the goal of most, if not all, manufacturers. In production environments where large quantities of product are produced in a relatively short amount of time, an inefficient quality control process can cause a substantial loss in production time, product, and revenue. Steel and paper production are typically high quantity operations and consequently would benefit from an efficient quality control mechanism. In each of these systems, the steel or paper product is typically produced in a sheet or web-like form along a production line at relatively high speeds. One of the key quality control parameters tested for these materials is strength. Testing the material strength of these products can be accomplished in many ways. Two general types of strength tests are destructive-type tests and non-destructive-type tests.
In destructive-type tests, the material is sampled and tests are conducted off-line to determine the various properties. Test time for off-line testing is relatively long, resulting in delays between sample collection and process changes. This delay allows substantial amounts of sub-standard material to be produced. Non-destructive tests, on the other hand, can often be performed on-line while the web or sheet-type material is being produced. One type of non-destructive testing used for web-like materials is ultrasonic testing. Ultrasonic testing is performed on-line enabling a continuous test of the mechanical properties of the material. In ultrasonic testing, sound waves are propagated through the material to determine the velocity of the sound waves. The velocity of the sound waves through the material correlates with the strength of the material.
In non-contact ultrasonic testing, an ultrasonic pulse is induced into the material and a stationery detection laser or interferometer is reflected onto the surface of the material to measure vibrations in the material due to the ultrasonic pulse. Laser interferometers detect surface motion of the material caused by the ultrasonic wave. The detection is normally accomplished by reflecting a laser beam from the surface of the material. The reflected beam is phase-shifted by variations in surface displacement from which desired measurements may be obtained by interfering with a stable xe2x80x9creferencexe2x80x9d beam. Surface variations may be caused by the ultrasonic wave or by other means, such as mechanical vibrations due to machinery, etc. Thus, the source of variations needs to be distinguished. The frequency component of the mechanical vibrations are usually in the 1-1000 Hz range whereas the frequency component of the ultrasonic signals are typically in the 1 MHz (megahertz) range. The different frequencies of the vibrations and the sought-after ultrasonic signals enable the machine vibrations to be easily filtered out so that the detection of the ultrasonic signal is readily obtained.
While mechanical vibrations may be easily filtered, noise induced by the texture of the surface can be problematic depending on the type of material being tested and/or on the speed at which the material passes under the detection laser beam. Non-contact testing has been successfully performed using a stationary laser beam in the production of steel sheets moving along a conveyor. The success of laser detection of ultrasonic sound in the production of steel can be attributed to the relatively slow moving speed of the steel on a conveyor and to the relatively smooth texture of steel. However, using on-line, non-contact measurements of ultrasound wave velocities in paper production is more problematic because the paper moves at a much higher speed and is more fibrous than steel. The high speed of production and very fibrous nature of the paper""s texture causes detection problems for a detection laser.
When a light from a stationery laser is reflected from a moving textured surface, such as paper, additional phase changes in the signal result from the textured surface. Specifically, the laser beam reflects on a fiber at one instance and, at the next instance, the laser beam reflects into the xe2x80x9cvalleyxe2x80x9d between fibers and, at the next instant, reflects again at the top of another fiber. The undulating laser beam reflection caused by the xe2x80x9chillsxe2x80x9d and xe2x80x9cvalleysxe2x80x9d of the paper texture produces a wave of noise that needs to be filtered out in order to detect the ultrasonic waves. However, filtering texture noise is not an option in fast moving materials because the frequency of the moving material can produce a noise component or texture signal that is in the 1 MHz range. The 1 MHz frequency range of the texture noise makes distinguishing the ultrasound wave virtually impossible because the ultrasound wave propagates at the same frequency. Additionally, the amplitude of the ultrasound wave is less than 0.1 micron whereas the amplitude due to surface deformation caused by the fibers of paper are approximately ten microns. Thus, not only does the frequency interfere with the filtering of the ultrasonic wave from the texture noise, but the texture noise blocks out the smaller ultrasonic signal.
Therefore, there is a need in the art for a non-contact ultrasonic testing system that is operative to distinguish ultrasonic signals from texture noise for relatively fast moving surfaces. In such a system, a need exists to reduce or eliminate motion-induced noise caused by the texture of a moving surface.
Generally described, the present invention provides a method to reduce motion-induced noise in the optical detection of ultrasonic signals in a moving sheet or body of material. The system of the present invention uses a detection laser to detect the ultrasonic signals traveling in the sheet of material.
More particularly, the present invention provides a method of reducing noise in the detection of an ultrasonic signal in a moving body of material by moving a detection laser beam along the surface of the material in the same direction as the direction of movement of the sheet of material. By moving the laser as described, the amount of textural noise induced in the detection of the ultrasonic signal is reduced. The method of the present invention may be used for many applications, such as in paper, steel or plastic making processes.
Another embodiment of the present invention includes a system for detecting ultrasonic signals in a moving body of material. In this system, a body of material moves in a certain direction along a defined path. An ultrasound generator directs an ultrasonic signal into the moving body of material. A scanner directs a laser beam onto the surface of the moving body and moves the laser beam along the surface of the moving body in the direction of movement of the moving body. A detection device detects a reflection of the laser beam from the surface of the moving body to detect the movement of the ultrasound signals in the moving body.
The scanner is preferably operable to move the laser beam at a speed that is approximately at the speed of the moving body. The scanner may be a galvanometer that is rotatable in the direction of movement of the moving body or may be any type of scanner suitable for directing the laser beam along the surface of the moving material. The operation of the scanner may be synchronized to begin when the ultrasonic signal is expected to arrive in the scanning path of the scanner.
Preferably, a photodetector is used to synchronize the movement of the scanner with the generation of the ultrasonic pulse. The photodetector detects the generation of a pulse from the ultrasound generator and generates a start timer signal. A timing circuit controls the movement of the scanner in response to receiving the start timer signal. The detection laser beam scans the body in the direction of movement, at a speed near that of the body, over a time period sufficient to detect the ultrasound pulse. The moving body of material may be a web of paper, steel, or other sheet material.
Thus, it is an object of the present invention to provide a method and system for improving the optical detection of ultrasonic signals in a moving body of material.
It is another object of the present invention to reduce the amount of texture-induced noise generated in the detection of ultrasonic signals in a moving body of material.
These and other objects will be readily apparent to those skilled in the art upon reviewing the accompanying descriptions and figures.